STRUCTURE AND DEFECTS OF MBE GROWN NB-AL2O3 INTERFACES

Single crystal niobium films were grown by Molecular Beam Epitaxy (MBE) on (0001)s sapphire substrates. Cross-sectional and plan-view specimens of the films were prepared so that the Nb-Al2O3 interface could be investigated by conventional transmission electron microscopy (CTEM) and high-resolution electron microscopy (HREM). The atomistic structure of the interface was identified by HREM. Regions of good matching and regions containing structural defects alternate at the interface. The defects can be described as misfit dislocations with no "stand-off" from the interface. The spacing of the defects corresponds to the 1.9% mismatch between the two lattices. In regions of good matching a fixed translational state between both lattices is established, which could be determined. Image simulations constructed with this translational state were in good agreement with experiment. In plan-view, periodic arrays of fringes were observed. These fringes are formed by a superposition of moire fringes and dislocation contrast. Weak-beam techniques were used to separate these two contributions.